The present invention relates to the structure of the optical multiplexing/demultiplexing device to multiplex or demultiplex the optical signals in the optical fiber communication circuits, and more particularly to the tree-coupler or star coupler structure of the optical multiplexing/demultiplexing device of multiple-port type where a plurality of optical output signals can be obtained when a single optical signal is input, or where one out of a plurality of optical signals can be input to send it to a single optical signal terminal.
Various types of optical multiplexing/demultiplexing devices with multiple input/output ports have been proposed for the optical fiber communications, and these devices are conventionally used to multiplex or demultiplex the optical signals in the optical fiber communications.
Many of them are of 2 by 2 matrix where two ports receive the inputs and the other two ports send the outputs since the 2 by 2 matrix type is easy to fabricate. When a system requiring a "m" by "n" matrix (m&gt;2 and n&gt;2) is built, two or more optical multiplexing/demultiplexing devices with 2 by 2 input/output ports are being used corresponding to the system size. One of them is the optical multiplexing/demultiplexing device wherein 2 by 2 input/output ports are built on a pair of substrates by polishing a pair of optical fibers on a pair of substrates before they are aligned.
FIGS. 3(a) and 3(b) show exploded cross-sectional views of a conventional optical multiplexing/demultiplexing device. FIG. 3(a) being shown cut across the optical axis thereof, and FIG. 3(b) being shown cut along the optical axis thereof.
Referring to FIGS. 3(a) and 3(b), the structure of the conventional optical multiplexing/demultiplexing device will be described hereafter.
In FIGS. 3(a) and 3(b), V-grooves 33 and 34 are trenched at the center on one sides of quartz glass substrates 31 and 32, respectively. Optical fibers 35 and 36 are installed into V-grooves 33 and 34, respectively. These optical fibers are fastened there by adhesives. Thereafter, clads 39 and 40 of optical fibers 35 and 36 are scrubbed off by polishing the optical fibers into the direction which is in parallel with the grooves 33 and 34 until cores 37 and 38 of the optical fibers 35 and 36 are just to be exposed, respectively. Substrates 31 and 32 are assembled in such a manner that the surface of substrate 31 on which V-groove 33 storing both core 37 and clad 39 is trenched faces the surface of substrate 32 on which V-groove 34 storing both core 38 and clad 40 is trenched and that cores 37 and 38 are aligned, and thereafter these substrates are fastened by setscrews or adhesives.
Optical fiber cores 38 and 39 can receive or send the optical power one another in the evanescent mode at a predetermined ratio of the sending to receiving powers.
If the gap between the centers of cores 37 and 38 of the respective optical fibers is trimmed by fine adjustment mechanism, the ratio of the sending to receiving powers can be set more precisely than that which is attained by any other type of the conventional optical multiplexing/demultiplexing device. In addition, the optical multiplexing/demultiplexing device shown in FIGS. 3(a) and 3(b) provides higher reliability and stability against shock and vibration, and exhibits the excellent performance over the wide temperature range.
The optical performance, however, depends on the accuracy of the work finished to construct the V-groove trenched on the quartz glass substrate. The quartz glass substrate has a linear expansion coefficient which resembles that of the optical fiber made of quartz, and thus an abrupt temperature change applies to the optical fiber such few tensile stress that it cannot break the optical fiber. Contrarily, the quartz glass substrate can be finished in three dimensions only by scrubbing the quartz glass substrate with abrasives. The quartz glass substrate can easily be broken by thermal shock during the work to fabricate it because of its hardness and brittleness, and V-grooves cannot be fabricated by scrubbing the quartz glass substrate with abrasives in most cases.
It is also known by a person skilled in the art that the use of the photoetching techniques which are generally used to work the quartz glass is limited in making a two-dimensional pattern with shallow trenches.
This type of optical multiplexing/demultiplexing device which is actually built on a pair of quartz glass substrates by scrubbing the quartz glass substrates with abrasives or by etching them in an etchent is quite a few in examples because of its difficulty in precisely making V-grooves on a pair of quartz glass substrates.
The conventional optical multiplexing/demultiplexing device with multiple input and output ports is thus made of a plurality of 2.times.2 optical multiplexing/demultiplexing devices, each having 2-input and 2-output ports, corresponding to the numbers of input and output ports required.
FIG. 4 shows an example of the structure of the optical multiplexing/demultiplexing device with one-input and 8-output ports (or 1 by 8 matrix).
Optical fibers coming out of seven optical multiplexing/demultiplexing devices 6a through 6g, each having 2-input and 2-output ports (or 2 by 2 matrix), are as shown in FIG. 4 connected together, to form a 1 to 8 star coupler, at points C1 through C6 by arc discharge alloying or through optical connectors in such a manner that device 6a is connected to devices 6b and 6c via points C1 and C2, device 6b is connected to devices 6d and 6e via points C3 and C4, and that device 6c is connected to devices 6f and 6g via points C5 and C6.
An optical input signal at input port P is divided and fed to eight output ports P1 through P8. For dividing an input signal into 16 output signals, 15 devices are to be used. For dividing an input signal into 32 output signals, 31 devices are to be used. For dividing an input into 64 output signals, 63 devices are to be used.
As described heretofore, a greater number of optical multiplexing/demultiplexing devices of 2 by 2 matrix type are required to build an optical multiplexing/demultiplexing device of "m" by "n" matrix type (where m&gt;2 and n&gt;2). The small-size, light-weight version is difficult to be built, if a greater number of input/output ports is required, because of its complexity in structure. Since the device size increases as the number of ports increases, the reliability and stability against shock and vibration go low with the number of ports.
The objective of the invention is to present an improved small-size version of the optical multiplexing/demultiplexing device having multiple ports which is fabricated by the precise work of the V-grooves wherein optical fibers are installed on each substrate, and which is fabricated in accordance with the thermal expansion compensation rule to minimize the possibility of breaking of the optical fibers due to the difference in the linear expansion coefficient between the optical fibers and substrates.